Mucus hypersecretion, goblet celt hyperplasia and metaplasia as well as submucosal gland hypertrophy are pathophysiological and histological hallmarks of chronic bronchitis. The activation of epidermal growth factor (EGF) signaling is believed to be responsible for many of these morphological changes of the airway epithelium. Cigarette smoke, the major cause of chronic bronchitis in human subjects, has also been demonstrated to induce mucin secretion and mucous cell hyperplasia, at least in part, via oxidative stress and EGF signaling. EGF is expressed in the airways, stimulates the EGF receptor (EGFR) and, as shown by us, is made from pro-EGF through cleavage by tissue kaltikreJn (TK). We have also shown that TK is secreted by submucosal glands together with hyaluronan, which inhibits the activity of TK. Hyaluronan also immobilizes bronchial TK at the epithelial surface, creating a pool of readily available, yet inactive TK at the airway surface. Reactive oxygen species (ROS) can cleave hyaturonan, thereby releasing activateded TK. In chronic bronchitis, ROS and active TK are elevated in the airway compared to normal subjects, possibly due to continued degradation of hyaluronan by ROS. We hypothesize that the increased availability of active TK in these conditions is a critical link between oxidative stress and EGF-mediated airway mucous cell hyperplasia. This proposal will test the hypothesis that ROS-mediated goblet cell metaplasia and submucosai gland celt hyperplasia occurs through a multi-step cascade beginning with hyaturonan degradation (initiation step) that results in sustained release of activated TK (priming step) and thereby increased availability of mature EGF (activation step) that in turn activates EGFR (signaling step). The specific aims are: 1) to confirm and extend our preliminary in vitro observations that ROS degrade epithelialbound hyaluronan and thereby release active TK that in turn cleaves pro-EGF to activate EGFR (proof of concept in culture); 2) to evaluate hyaluronan degradation, TK activation and pro-EGF processing in airways obtained from patients with chronic bronchitis (proof of concept in human disease); and 3) to examine whether TK-mediated pro-EGF cleavage regulates submucosal gland cell proliferation (relevance of concept). The proposed experiments will thus examine a new link between oxidative stress and chronic bronchitis and may identify new approaches to prevent or even reverse such changes in the airway mucosa.